Oxidation-resistant, highconductivity material



June 23, 1964 J. G. DARRAH ETAL OXIDATION-RESISTANT, HIGH-CONDUCTIVITY MATERIAL Filed May 25, 1961 INVENTO RS JAMES G DARRAH ROBERT' W- HARRISON dw. fMw

ATTORNEY United States Patent 3,137,928 Patented June 23, 1964 ice 3,137,928 OXIDATION-RESISTANT, HIGH- CQNDUCTIVITY MATERAL James G. Dai-rah, Wethersfield, and Robert W. Harrison,

Portland, Conn., assignors to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Filed May 23, 1961, Ser. No. 112,022 4 Claims. (Cl. Z9-182.5)

In heat-transfer systems, particularly those utilizing high-temperature, very-corrosive liquid metals, a highthermal-conductivity, oxidation-resistant material is required Which can be rolled into thin sheets. Further, these sheets should have sutlicient ductility to allow blanking into discs and other shapes required in radiators, heat exchangers, and plates or tubes having extended surfaces.

It is a principal object of this invention to provide a material which meets all of these requirements to a high degree.

Although pure nickel possesses the required high thermal conductivity, all previously known alloying elements added to nickel that improved its oxidation resistance acted at the same time to very seriously impair its thermal conductivity.

We have discovered that certain ceramic phases, although insoluble in nickel, when properly dispersed throughout the nickel, markedly increase the oxidation resistance of nickel with little or no reduction of the thermal conductivity of the nickel.

Among the ceramic materials which, when nely dispersed as submicron-size particles throughout nickel, showed this effect were alumina (A1203) and nickel-alumina spinel (NiAl204). The quality of the dispersion is critical in achieving the desired properties, since a course dispersion of the same composition does not impart the oxidation resistance to the nickel as does the more finely dispersed structures.

Although A1203, added in quantities of 8-10 volume percent, was as effective in imparting oxidation resistance to nickel as was volume percent NiAl2O4 in the assintered condition, the resulting structure could not be fabricated into thin sheets. However, the cermet structure containing NiAl2O4 could be cold-rolled into thin sheets and had suicient ductility to allow blanking the sheets into discs. The cold-rolling further improves the nature of the dispersions by breaking up agglomerates and thereby increases the oxidation resistance. The coldrolling should preferably be accomplished with intermediate anneals at 1500 F. in argon.

To achieve these tine dispersions of the insoluble oxide additions to the nickel, much attention must be given to the powder metallurgical preparation processes. Both the nickel and the ceramic components to be added must be blended in the linely divided state. A tine-particle nickel powder (about 5 microns) obtained by reduction of nickel carbonyl, and a iinely divided (sub-micron size) NiAl204 made by reacting equal molar ratios of NO and A1203 at 2400 F. for two hours, are ball-milled together in a ceramic mill containing alundum balls for twentyfour hours. Care must be taken that the materials are absolutely dry. After mixing, the composition is cornpacted at twenty tons per square inch and sintered in vacuo at 2400" F. for four hours,

An examination of the single figure shown in the drawing reveals two inection points in the continuous weight gain data for oxidized, unalloyed nickel sheet. This behavior indicates that a critical thickness of oxide forms, becomes semiadherent, and develops fissures opening new avenues for oxygen to come in contact with the freshly exposed nickel surface. The increase in oxidation rate at the inflection points at approximately thirty hour intervals and postulated behavior of the oxide film may be correlated with the metallographic evidence, which clearly shows three oxide layers developed in an oxidation test at 1650" F. for one hundred hours. It may be seen from examination of the drawing that the improved oxidation resistance of NiAl204-containing material may be attributed to two observed effects: (1) lower initial oxidation rates and, (2) the development of adherent scales.

lt should be noted that, in addition to the oxidation resistance imparted to the nickel by the insoluble oxide addition, there is also a welcome increase in the strength of the nickel.

From the above it will be evident that we have provided a high-conductivity, oxidation-resistant material which can be cold-rolled into thin sheets having good ductility, allowing it to be fabricated for extended-surface radiator applications.

We claim:

1. A high-conductivity oxidation-resistant sintered material having room temperature ductility and consisting essentially of 5 micron size particles of Ni blended with 5 v/o of submicron size particles of NiAl2O4.

2. A high-conductivity oxidation-resistant sintered material having room temperature ductility, and consisting essentially of a blended composition of 5 micron size particles of Ni matrix and an insoluble ceramic phase of about 5 v/o of submicron size particles of NiAl2O4.

3. The method -of preparing a high-conductivity oxidation-resistant sheet material consisting of Ni-S v/o NiAl204 which consists in preparing a line-particle nickel powder of about 5 microns fineness by reduction of nickel carbonyl, preparing sub-micron size particles of NiAl2O4 by reacting equal molar ratios of NiO and A1203 at 2400 F. for twenty-four hours, blending the two by ball-milling them together in a ceramic mill, compacting the composition in a multiton press, sintering in vacuo at 2400 F. for four hours, and cold-rolling into sheet form with intermediate anneals at l500 F. in argon gas.

4. The method of preparing a high-conductivity oxidation-resistant thin sheet material which consists in blending a mixture of Ni in finely divided powdered state of about 5 micro-ns size with a small addition of a lineparticle insoluble nickel-oxide aluminum oxide spinel of submicron size by ball-milling the two together in a dry state in a ceramic mill, compacting the composition by pressure, sintering the compressed material, and coldrolling with intermediate anneals at l500 F. in argon gas.

References Cited in the lile of this patent UNITED STATES PATENTS 2,698,990 Conant et al Jan. 11, 1955 2,852,367 Goetzel et al. Sept. 16, 1958 2,910,052 Wood et al Jan. 31, 196,0 2,961,325 Mayfield Nov. 22, 1960 3,019,103 Alexander et al. Jan. 30, 1962 

1. A HIGH-CONDUCTIVITY OXIDATION-RESISTANT SINTERED MATERIAL HAVING ROOM TEMPERATURE DUCTILITY AND CONSISTING ESSENTIALLY OF 5 MICRON SIZE PARTICLES OF NI BLENDED WITH 5 V/O OF SUBMICRON SIZE PARTICLES OF NIAL2O4. 